Heat oven to 375° F. In a large mixing bowl, cream the butter until fluffy. It will be difficult to cream 2 tablespoons of butter in an electric mixer, so your best bet may be to use a wooden spoon for the first part, stirring, mashing, and fluffing hard for half a minute to a minute. Add the sugar and mix well. Then add the egg yolks, one at a time, beating each in thoroughly before adding the next. Continue beating until light and fluffy. Stir in the flour and salt and combine well. Then stir in the lemon zest, lemon juice, and milk.
In a separate bowl, beat the egg whites to stiff peaks (see note). Fold them gently into the mixture. The egg whites will have a tendency to float; this is fine.
Pour the mixture into a prepared 9-inch pie shell. Bake for 40 minutes or until the top is browned and the filling is set.